Long ago, while working on the Echelon Reference Series, I decided that I really wanted to separate ability definitions from the game elements that granted them.
For instance, I didn’t like to see ‘evasion’ defined in the rogue class, and then other classes (monk and ranger) saying “gain evasion, as the rogue ability”, or defining it again (potentially inconsistently). What I really wanted was ‘evasion’ to be defined once, and then everything that granted it (and everything that gained it such as animal companions) just points at that definition.
After examining the cavalier and ranger, I realized that many abilities could be gained by more than one path, or via feats. It is easy to imagine that a druidic path could grant favored terrain, or that a desert nomad might have a single favored terrain.
Because a single ability can be gained in multiple ways, it seems reasonable that the abilities should each be defined once, and the way the abilities are granted refer to these. A ranger gains a favored terrain at each ranger level, and the bonuses increase every other level. A druid’s wildshape is pretty good, but let’s give them a bit of a bump in the form of a favored terrain every other level and the benefits according. A desert nomad gains only one favored terrain (desert, obviously) but it improves at each level.
The abilities don’t change, but paths and feats can grant them in different ways.
Spell lists can be similarly handled, then reused. A tradition and a domain might both grant expert fire magic (same spells in both cases), but might change the mode of casting somewhat (domain requires a divine focus, the tradition might give the option of increasing effort to increase range or area of effect). The death master domain and the necromancer master path might both give the master death spell list, but again change how they cast.
In all cases there could be different associated powers, or the same ones (less likely), or a mix (probably most common). The necromancer and the death master domain might both grant the rebuke undead ability at the master tier.
As a data professional, I’m a big fan of DRY: “Don’t Repeat Yourself”. Centralizing and standardizing definitions, then defining more complex elements in terms of those standard definitions, will help simplify things.
Speaking of standardizing definitions, I realize I need to normalize and standardize my terminology. I find I’m not always using words exactly the same way each time.